For me, the most important part about a seatpost is its adjustability. The balance between weight and strength comes next...

Take the existing QR seat clamp off, and see if its diameter is stamped inside. They're often marked. Otherwise, the easiest way is to a) look up the bike's spec online or b) have you shop measure the outside of the frame with calipers.

As for bar width, the Salsa bar on my single speed started life as a 710mm bar, and is now ~660 mm wide. I cut it down in 1/2 centimeter increments until I liked the way that it felt and stopped hooking it on trees